Sunday, October 16, 2005

Wow, what a month: AMSAT, OFDM, etc.

The month since my last post has seen a whirlwind of activity. I have been working on several things and you can expect to hear more about all of this as time passes.

In August and September, I joined a conversation with John Stephensen, KD6OZH about doing some really nice coding for OFDM based modems for communications systems for amateur radio and even outside of amateur radio. Frank Brickle and I are building our Near Vertical Incidence Skywave antenna systems to use the 7 MHz (40 meter) amateur radio band to some experimentation on this system.

We will be doing our first experiments with the SDR-1000 because of the ease of using it for wideband HF communications. We will be implementing a concatenated code system (two forward error correcting codes back-to-back) in conjunction with a permutation system for the data and then encoding this data for tranmission in a system called Orthogonal Frequency Division Multiplexing (OFDM). The basic reasons for such a system is that in makes use of diversity in time and frequency to spread out the pain of transmitting data over bad paths. It has been demonstrated in many places how effective this type system can be on the horrible paths of HF (shortwave).

In addition to the work briefly described above, I have begun a serious study on the tranmission protocol for such a system. There has been tremendous progress in systems that transmit data and then retransmit data if the receiving misses all or part of the data. We believe we can implement an automatic request for retransmission (ARQ) system that can achieve rates that approach 90% efficiency and waste a minimum amount of time doing the transmission protocol. Lin/Costello's new edition of Error Control Coding has just been released and their is an absolute spectacular discussion in an entire chapter with a huge bibliography on this topic.

At the annual meeting of AMSAT, NA Software Defined Radios were a major topic. Tom Clark and I have been working on a system we jointly proposed with Phil Karn two years ago and we have made steady progress in studying the system (see http://www.gpstime.com for the CC Rider papers). In the two years we have been pursuing this scheme, we have done a fair (C+) job of selling the system. Matt Ettus of GnuRadio and USRP fame

did a better job in 45 minutes than we have done in two years of selling the entire concept. He proposed a hardware system for doing the communications and really blew the audience out of their socks. With the system proposed by Matt, we would be able to build a fairly complex text messaging system that would need a "paper clip" for an antenna!

I was first alternate to the board of directors for AMSAT this year. It was interesting to do be in on these conversations this year as there is a lot of new blood in the AMSAT board. I was a member of the board of directors a decade ago and more and it brought back pleasant and unpleasant memories of the wrangling that goes on in these meetings. Anyway, the organization asked me to be its Vice President of Engineering. This is one of the corporation official offices (in the by-laws) and will require me to change modes a bit. Thankfully Jan King and Karl Meinzer (google for many pages) will be available for technical consulation. Please visit: